Mainsail Flotation

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Mainsail Flotation

Postby jdoorly » Wed Dec 01, 2010 6:39 pm

I'm researching the concept and implementation of sewing flotation material into the head of my mainsail, and I've found very little except for the description by Geoff Heath in Small Craft Advisor #63 (May/June 2010) where he says he did it. In his drawing the flotation appears to go from near the headboard to near the top batten (that's 50 leech inches on my sail) but does not give any specs like thickness or cubic inches or pounds of positive bouyancy. Other than that all I can do is ask for help, HELP!

I'm thinking someone said they put a throwable seat cushion or a life preserver under the masthead to keep the capsized boat from turtling. Does someone else remember this?
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Postby GreenLake » Wed Dec 01, 2010 7:38 pm

Heres's my thinking:

If you've ever tipped your boat with the mast raised, you know that with a 20+ ft lever, it takes just a few pounds to balance the boat when it's on edge.

So, if your mast doesn't have positive flotation, you'd need to support half its weight (the hull will hold the other half) plus some extra (neglecting the sails, as they are close to neutral).

So, roughly, if you add enough flotation to correspond to the weight of the mast, or even a bit less, then you should be in the right ball park.
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Postby jdoorly » Thu Dec 02, 2010 1:42 am

So, whats a henway, noway, I mean mastweigh? I checked Dwyer and a DM3 extrusion is 0.971 lb/ft so that's about 23 lb. I couldn't find weights for rigging, so I'll guess 15lb, so that's a total of 38 lbs for the rigged mast. Divide by 2= 19lbs for equal weight distribution.

My pfd is rated for use by people > 90 lbs, i.e. for use by me at 170 lbs, and has about 17 lbs of positive bouyancy. 17/170= 10%, and 10% of 19 lbs= 1.9 lbs. So does that mean only 2 lbs of positive bouyancy will float the mast?

My pfd has a volume of 525 cu in. 2lb/17lb = 12%, so .12x525= 63 cu in
Which is only 1"x8"x8". That sure doesn't seem right! What did I do wrong?
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Postby GreenLake » Thu Dec 02, 2010 3:00 am

Let's round.

20lbs will float half your mast and rigging.

1 cuft of foam will float 60lbs in saltwater.

To reach 20 lbs you will need 1/3 of cubic foot or 24"x24"x1".

If you seal your mast, or have foam inside it, then you can offset that volume from the external flotation.

The Dwyer DM3 has a closed section of about 2.15" diameter which is approximately circular. Assuming a .1 reduction due to wall thickness, the volume of one foot of this extrusion is therefore pi /4 * (2.05/12) * (2.05/12) cubit feet, or a bit more than pi/144.

Accounting for the weight of the extrusion, each foot of a foam filled extrusion adds 60 * pi / 144 - 0.971 pounds of buoyancy in saltwater, or .4 lbs. The full length of the 25' mast would float up to an additional 10 lbs of rigging. Which is what Brad reported.

The problem is that if the mast is not flat in the water, not all of it is immersed, and the hull can start to angle, which gives it a tipping moment. The cushion, or the added flotation should be aimed at countering that, I think.

Does that make sense, and do you agree with my estimate?

You probably don't need an additional 20lbs of flotation. So perhaps 18 x 18 x1 will be enough of a flotation panel.

Your calculation for the personal floatation doesn't apply, because it is based on the very small negative buoyancy of the human body relative to its dry weight.
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Postby jdoorly » Thu Dec 02, 2010 5:51 pm

Mmmmmmmmmmmmmmmmmmm, I love pi! 'specially aple.

I can see that my body is 86% water as opposed the metal mast and 20 lbs of bouyancy seems feasible. I'm sure you could balance the boat on its' side with the slightest touch near the top of the mast and that you need a safety margin in the flotation to overcome wind, wave, and current. and I'm liking the idea of a 1x18x18" panel and some foam in the mast either to add flotation or more importantly to keep water out so you can right the boat.

But I'm having trouble with your derivation of V= pi*(r*r)*h, and what's worse I'm getting different results...
r*r= 1.025x1.025= 1.05
r*r*h= 1.05x12= 12.6
pi*(r*r)*h= 3.14159x12.6= 39.6 cu in per foot or 0.023 cu ft per foot

Checking by dividing 39.6 cubes by two possible dimensions, 1 and 12 leaves 3.3 cu in per inch. and pi*r*r says area = 3.14*1.05*1.05= 3.3 sq in.

However, this was a display of the farthest extent of my math 'skills'. It's fun to get the wheels turning again but me and math have never been on the same side, so there's plenty of room for error.
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Postby dbk0630 » Thu Dec 02, 2010 6:20 pm

Based on the premise that it never hurts to do some benchmarking... the mainsail floatation available from the factory for Flying Scots is designed to provide 30lbs of bouyancy for the top of the mast... at a price of $175!

I won't pretend to understand the math that might be involved... but given the specs for a Flying Scot...
Sail Area, main & jib . . . . 191 sq. ft./17.65 sq. m.
Weight, all up . . . . . . . . 850 lbs./385 kg.
... it seems you might/could scale your design based on the FS as a reference. It might alos be worth calling the FS factory to get the specific dimensions for theirs (and perhaps some advice).

And since you got me curious I found a video showing a capsize and recovery of a Flying Scot (one with the masthead floation installed). The video makes righting the boat look like a piece of cake... at least for two people:

http://www.nockamixonsailclub.org/content/video-flying-scot-capsize-and-recovery

Anywho, good luck with your project... I look forward to hearing what you end up developing.
Dave K.
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Postby seandwyer » Fri Dec 03, 2010 11:28 am

OK - I don't understand how this was so easy. I watched the video a few times and really, this boat was righted by one person. The man in the water doesn't seem to do anything - if anything he made righting even harder, just letting the boat scoop him up out of the water. I've never capsized, but everyone seems to say that righting a DS is nearly impossible. Given that the DS and FS are similar boats - but the FS larger and heavier with more sail area, why is it seemingly so easy to right? Is it really all in the mast head flotation - because if so, I think we all need to look into this - maybe having a company make many of them all at once so as to bring the price down. I'd definitely be interested even at 100 to 175 - it would be worth it.

One thing that I noticed though was that the FS seemed to really float on its side high out of the water. Having never gone over, how does the DS compare? Maybe not so high and thats the difference?
Sean
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Postby jeadstx » Fri Dec 03, 2010 12:15 pm

Watched the video also. When I capsized last March, my DS2 sat on it's side about the same as the one in the video. I was able to keep my masthead from sinking too far with the help of a young man from a passing boat. The mast was starting to fill with water, so I sealed it and added some flotation near the top before my next sail, fortunately I haven't had the occassion to try it (in other words, I haven't capsized again). I swam my boat (holding on to the centerboard) to shallow water (4-5 feet deep) where I could get leverage with the bottom of the lake to right the boat. After thinking about it, had I righted it in deep water, I don't know if I could have gotten back in the boat, I'm a big man, not young, and agile anymore. I think that had the masthead not gone under a couple feet, that the boat was rightable by one person in deep water. The resistance of water against the sail seem to be the main problem in righting. I had released the mainsheet, but the sails still offered resistance to righting. I have to say that the DS really looks big when floating in the water looking up at the bottom.

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Postby GreenLake » Fri Dec 03, 2010 7:20 pm

I think our numbers agree rather nicely. You arrived at:

jdoorly wrote:pi*(r*r)*h= 3.14159x12.6= 39.6 cu in per foot or 0.023 cu ft per foot.


If you multiply that with 60, you get 1.38. That's 1.38lbs / ft of flotation for the volume of 1 ft of mast section in fresh water (60 lbs / cuft).

If you subtract the value of .971 you get .409 lbs which is the excess flotation over the weight of the mast for each foot of mast profile (which is the .4 I wrote about).

If half your mast (above the cuddy top) is pushed under that's about 10ft, or nearly 14lbs of flotation. Against your estimate of 38/2 = 19lbs for the part of the weight of mast and rigging not supported by the hull.

If these estimates mean anything, they would indicate that the DS just has to turtle unless you can provide flotation at the mast top, or are able to quickly provide a righting moment (e.g. standing on the CB).
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Postby jdoorly » Fri Dec 03, 2010 11:45 pm

Thanks dbk0630 for the benchmark, that's really helpful. I used my graticulated eyelashes to scale the flotation patch on a Scot in the video. By the way, they must be setting a record at under 30 seconds to recover! The patch appears to be a trapezoid with b1=8" b2= 24" and ave h=27". Area= 1/2*(b1+b2)*h= 432 sq in. Making the foam 2" thick would give 864 cu in which happens to be .5 cu ft and 30 pounds of up.

Sean, I'll most likely be doing this project in the next couple weeks and I'll write up a Bill of Materials and where to get stuff and instructions. This project should be easy; cut some sailcloth, apply some doublesided tape and the flotation and apply the patch to the sail. If your anal like me you can add a simple straight stitch in. I do plan on leaving the bottom side open but with a fastening so I can play with the amount of flotation, when the water gets warm. If you do it yourself the sail cloth is about $15 a yard, I got 2 cheap pfd's for $8 each for 34 lbs of flotation, glue or tape is about $5.

jeadstx, if say 5 feet of your masthead goes under and that 5 feet fills with water that only weighs about 7 lbs, but that 7 lbs plus 5 lbs of mast weight is at the end of a 24 ft lever making a force of 288 ft lbs. Thats a lot of inertia to overcome before the sails start coming up!

GreenLake, yea, it's kinda scary not to put flotation in the mainsail. And many thanks for the geometry lesson.

At this point I'm thinking the FS patch provides a certain excess safety margin that is not unwanted, but the flotation is stealing real estate from the aerodynamically efficient part of the masthead. I'm wondering what the most efficient foam shape is, I don't think it's a flat panel. I would like to have a panel that would float 25 lbs (Ex. 2"x19"x19" ), but I wonder if the panel should be more like 3" wide (on each side, total 6") x8" tall x24" but shaped in a foil? The front could be blunt because its behind the mast but the tail should taper.
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Postby jdoorly » Mon Dec 06, 2010 9:20 pm

Paging through an old book "Sailing Theory and Practice" I was reminded that the head of a mainsail is subject to induced drag (i.e. air from the windward side gets sucked over to the leeward side reducing the forces of good, pie, and the Amer-I-can way, and causing a nice foil shape to be useless. So I'm back to thinking about flat panel flotation as the optimum.

I'm a little uncomfortable with a single panel on one side of the sail and would rather add flotation patchs symetrically to both sides of the sail so it doesn't limp on one tack.

The mainsail still needs to flake neatly on the boom when not in use and also fit in a sailbag and a single big panel would preclude that. So, now I'm thinking of dividing the big panel into a 2 slightly seperated trapezoids and a 2.5" area between the panels would allow the sail/panels to fold neatly. My slides on the mains' luff are 18" apart and the folding/flaking action needs to occur halfway between the slides. If the top of the highest panel starts 4.5" down from the top of the headboard and ends 10" (h) vertically below and the trapezoids' b1=6.75" and b2=15.25 and they are 1.5 inch thick on both sides the sail bouyancy is 10.8 lbs. If the next trapezoid starts 2.5 inches below with a b1=17.625" and b2=31.25" and h=15" and they are 0.75 inch thick on both sides the bouyancy is 19.08 lbs for a total of 29.95 lbs.

I expect to taper the foams' sides on the leech in the last 2" which will loose about .5 lb of bouyancy. Also the edges between the boards where they will fold need to be rounded. I'm thinking about making 1/8" cuts 3/4s through the foam board every inch or so to make the board conform better to sail shape. BTW, if you got lost in the numbers above the upper panel is 1.5" thick on each side and the larger lower panel is .75" on each side because the higher area is less efficient converting wind into wake.

From earlier discussions you probably don't need more than 10 to 15 pounds of bouyancy at the masthead but having more makes me feel warm and fuzzed, and if I'm capsized in 6 footers and a gale blowing up my bustle I hope it's enough. Maybe make the upper floation permanent and the lower could be attached in heavier weather?

I said before I got 2 cheap pfds for the flotation, but I found even cheaper foam at Home Depot. They have Owens Corning Foamular 3/4" x 24" x 96" Styrofoam Insulation Board for $7.20 each. One of these boards has 1 cu ft of foam or 60 lbs flotation, but it is pink (actually I think styrofoam is rated at 55 lbs per cu ft). Also I'm thinking about using a much lighter fabric then 5 oz sailcloth, not sure what, maybe ripstop nylon.

BTW, What kind of foam did you guys put in your mast?
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Postby algonquin » Tue Dec 07, 2010 12:50 am

jdoorly wrote:BTW, What kind of foam did you guys put in your mast?


I just copied part of this this from a prior post. I don't use any foam but seal the mast. Here is how I approached the problem of adding flotation to the mast.

I disassembled the mast and all the attached hardware from the tabernacle on up. I fabricated a cover plate inserted just below the mast head pulleys. After I applied a high quality clear silicone to each opening and reassembled the hardware. The silicone is not visible after the reassembly. I float tested the mast assembly and it is highly buoyant. After 2 seasons of use I suspected there may be some condensation in the mast so I removed the tabernacle plate and much to my joy found no condensation accumulation in the mast.

As far as spray foams go I know that many use the canned foam insulation filler available at most hardware stores. You do have to be careful that you don't have this stuff oozing out of every available orifice on the mast and especially the mast head and pulleys. It expands significantly while setting up. Another drawback is not knowing really how much of the mast is actually filled. I suppose you could remove most of the hardware attachments and squirt the filler in at various locations along the mast to help insure coverage. Brad
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Postby GreenLake » Tue Dec 07, 2010 2:39 am

OK, I can't resist playing with numbers a bit more.

The CB on a Flying Scot is 105 lbs. On the DS it cannot be mort than 25 lbs. So this is equivalent to having an extra 75lbs at the middle of the CB. Also the mast is (a tad) shorter in relation to boat length, on a DS a mast scaled to comparable length would be 2' shorter.

Anyway, 75 lbs at 2' from the hull is 150 ft lbs, or equivalent to about 5-6 pounds of lift at the tip of the mast (at something like 28' leverage for the FS, more for the DS with its shorter mast).

Even with adding this extra amount to your properly scaled mast flotation, you will only get to the same stability while in the water. You still won't get the benefit of the additional righting moment, once the mast goes out of the water.

What you see in the video is how much the boat resists turtling (as the crew climbs up on the inside, he would force the mast down normally). But you also see how easy it is to create the righting moment to lever up the boat from the horizontal position.

Even with masthead flotation, that's going to remain harder on a DS than on a FS, because of the ballasted CB and shorter (relative) mast of the FS.

Anyway, by now I'm getting really, really curious to see how your project works out in a life-like test. Can't wait for your to report back. With video! 8)
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Postby MrPlywood » Sat Dec 11, 2010 9:08 pm

re: mast flotation - I've thought about the spray-in foam too. I wonder, though, about the rigidity of the cured foam and whether the mast flex would be affected and/or would the mast flexing break the foam down. Some of the hardware variety foam filler sets up quite stiffly, while others retain some flex. I have no idea if they have any actual flotation properties though. The marine grade foam - true flotation foam - is pretty pricey. Seems like it would make more sense to stuff some noodle-variety foam into the mast from the top so the flotation would be where it's needed most.

I watched the Flying Scot vid. Pretty amazing. That thing floated so high water didn't have a chance to get into the cockpit. I plan to try a controlled capsize next summer so I can find out what will actually happen when and if I go over.
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Postby Baysailer » Sat Dec 11, 2010 10:07 pm

Last summer I did an experiment where I covered the upper stays with pool noodles, then intentionally capsized. The noodles kept the mast above the water line and the hull sat about at coaming. I righted it myself which was good because I had no backup. Once upright there was water but not unbailable by any means. I was expecting more.

All this was done on a dead calm day with bare poles and the DS completely cleaned out and tied to the mooring. So there was no relation to a real situation. Doesn't prove much except that pool noodles on the uppers look real goofy. Maybe could use red and green noodles for port and starboard.

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